proplists

Support functions for property lists

Property lists are ordinary lists containing entries in the form
of either tuples, whose first elements are keys used for lookup and
insertion, or atoms, which work as shorthand for tuples {Atom, true}. (Other terms are allowed in the lists, but are ignored
by this module.) If there is more than one entry in a list for a
certain key, the first occurrence normally overrides any later
(irrespective of the arity of the tuples).

Property lists are useful for representing inherited properties,
such as options passed to a function where a user may specify options
overriding the default settings, object properties, annotations,
etc.

Two keys are considered equal if they match (=:=). In other
words, numbers are compared literally rather than by value, so that,
for instance, 1 and 1.0 are different keys.

property() = atom() | tuple()

append_values(Key, ListIn) -> ListOut

Key = term()

ListIn = ListOut = [term()]

Similar to get_all_values/2, but each value is
wrapped in a list unless it is already itself a list, and the
resulting list of lists is concatenated. This is often useful for
"incremental" options; e.g., append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}]) will return the list
[1,2,3,4].

expand(Expansions, ListIn) -> ListOut

Expands particular properties to corresponding sets of
properties (or other terms). For each pair {Property, Expansion} in Expansions, if E is
the first entry in ListIn with the same key as
Property, and E and Property
have equivalent normal forms, then E is replaced with
the terms in Expansion, and any following entries with
the same key are deleted from ListIn.

For example, the following expressions all return [fie, bar, baz, fum]:

Note that if the original property term is to be preserved in the
result when expanded, it must be included in the expansion list. The
inserted terms are not expanded recursively. If
Expansions contains more than one property with the same
key, only the first occurrence is used.

See also: normalize/2.

get_all_values(Key, List) -> [term()]

Key = term()

List = [term()]

Similar to get_value/2, but returns the list of
values for all entries {Key, Value} in
List. If no such entry exists, the result is the empty
list.

See also: get_value/2.

get_bool(Key, List) -> boolean()

Key = term()

List = [term()]

Returns the value of a boolean key/value option. If
lookup(Key, List) would yield {Key, true},
this function returns true; otherwise false
is returned.

See also: get_value/2, lookup/2.

get_keys(List) -> [term()]

List = [term()]

Returns an unordered list of the keys used in List,
not containing duplicates.

get_value(Key, List) -> term()

Key = term()

List = [term()]

Equivalent to get_value(Key, List, undefined).

get_value(Key, List, Default) -> term()

Key = term()

List = [term()]

Default = term()

Returns the value of a simple key/value property in
List. If lookup(Key, List) would yield
{Key, Value}, this function returns the corresponding
Value, otherwise Default is returned.

See also: get_all_values/2, get_bool/2,
get_value/2, lookup/2.

is_defined(Key, List) -> boolean()

Key = term()

List = [term()]

Returns true if List contains at least
one entry associated with Key, otherwise
false is returned.

lookup(Key, List) -> none | tuple()

Key = term()

List = [term()]

Returns the first entry associated with Key in
List, if one exists, otherwise returns
none. For an atom A in the list, the tuple
{A, true} is the entry associated with A.

See also: get_bool/2, get_value/2,
lookup_all/2.

lookup_all(Key, List) -> [tuple()]

Key = term()

List = [term()]

Returns the list of all entries associated with Key
in List. If no such entry exists, the result is the
empty list.

Passes ListIn through a sequence of
substitution/expansion stages. For an aliases operation,
the function substitute_aliases/2 is applied using the
given list of aliases; for a negations operation,
substitute_negations/2 is applied using the given
negation list; for an expand operation, the function
expand/2 is applied using the given list of expansions.
The final result is automatically compacted (cf.
compact/1).

Typically you want to substitute negations first, then aliases,
then perform one or more expansions (sometimes you want to pre-expand
particular entries before doing the main expansion). You might want
to substitute negations and/or aliases repeatedly, to allow such
forms in the right-hand side of aliases and expansion lists.

property(PropertyIn) -> PropertyOut

Creates a normal form (minimal) representation of a property. If
PropertyIn is {Key, true} where
Key is an atom, this returns Key, otherwise
the whole term PropertyIn is returned.

See also: property/2.

property(Key, Value) -> Property

Key = Value = term()

Property = atom() | {term(), term()}

Creates a normal form (minimal) representation of a simple
key/value property. Returns Key if Value is
true and Key is an atom, otherwise a tuple
{Key, Value} is returned.

See also: property/1.

split(List, Keys) -> {Lists, Rest}

List = Keys = [term()]

Lists = [[term()]]

Rest = [term()]

Partitions List into a list of sublists and a
remainder. Lists contains one sublist for each key in
Keys, in the corresponding order. The relative order of
the elements in each sublist is preserved from the original
List. Rest contains the elements in
List that are not associated with any of the given keys,
also with their original relative order preserved.

substitute_aliases(Aliases, ListIn) -> ListOut

Aliases = [{Key, Key}]

Key = term()

ListIn = ListOut = [term()]

Substitutes keys of properties. For each entry in
ListIn, if it is associated with some key K1
such that {K1, K2} occurs in Aliases, the
key of the entry is changed to K2. If the same
K1 occurs more than once in Aliases, only
the first occurrence is used.

Example: substitute_aliases([{color, colour}], L)
will replace all tuples {color, ...} in L
with {colour, ...}, and all atoms color
with colour.

See also: normalize/2, substitute_negations/2.

substitute_negations(Negations, ListIn) -> ListOut

Negations = [{Key, Key}]

Key = term()

ListIn = ListOut = [term()]

Substitutes keys of boolean-valued properties and
simultaneously negates their values. For each entry in
ListIn, if it is associated with some key K1
such that {K1, K2} occurs in Negations, then
if the entry was {K1, true} it will be replaced with
{K2, false}, otherwise it will be replaced with
{K2, true}, thus changing the name of the option and
simultaneously negating the value given by
get_bool(ListIn). If the same K1 occurs more
than once in Negations, only the first occurrence is
used.

Example: substitute_negations([{no_foo, foo}], L)
will replace any atom no_foo or tuple
{no_foo, true} in L with {foo, false},
and any other tuple {no_foo, ...} with
{foo, true}.